Imprint of inflation on galaxy shape correlations

TL;DR

This paper explores how intrinsic galaxy shape correlations can serve as a new probe of primordial non-Gaussianity and inflationary physics, especially anisotropic features, offering complementary insights to traditional galaxy counts and CMB measurements.

Contribution

It derives all-sky two-point correlations of galaxy shapes considering non-Gaussianity with angular dependence, highlighting the sensitivity to anisotropic inflationary signals and superhorizon modes.

Findings

Future surveys can constrain anisotropic non-Gaussianity similarly to CMB.

Galaxy shape correlations are sensitive to long-wavelength anisotropic modes.

The analysis links galaxy shape alignments to inflationary physics beyond isotropic models.

Abstract

We show that intrinsic (not lensing-induced) correlations between galaxy shapes offer a new probe of primordial non-Gaussianity and inflationary physics which is complementary to galaxy number counts. Specifically, intrinsic alignment correlations are sensitive to an anisotropic squeezed limit bispectrum of the primordial perturbations. Such a feature arises in solid inflation, as well as more broadly in the presence of light higher spin fields during inflation (as pointed out recently by Arkani-Hamed and Maldacena). We present a derivation of the all-sky two-point correlations of intrinsic shapes and number counts in the presence of non-Gaussianity with general angular dependence, and show that a quadrupolar (spin-2) anisotropy leads to the analog in galaxy shapes of the well-known scale-dependent bias induced in number counts by isotropic (spin-0) non-Gaussianity. Moreover, in presence of non-zero anisotropic non-Gaussianity, the quadrupole of galaxy shapes becomes sensitive to far superhorizon modes. These effects come about because long-wavelength modes induce a local anisotropy in the initial power spectrum, with which galaxies will correlate. We forecast that future imaging surveys could provide constraints on the amplitude of anisotropic non-Gaussianity that are comparable to those from the Cosmic Microwave Background (CMB). These are complementary as they probe different physical scales. The constraints, however, depend on the sensitivity of galaxy shapes to the initial conditions which we only roughly estimate from observed tidal alignments.